As mentioned above, many different types of downhole activities in well bores are conducted through utilization of the energy developed by first or second order initiation of an explosive substance. For example, in the completion of wells a blast joint incorporating multiple shaped charges is detonated at a desired downhole depth causing the shaped charges to perforate the well casing and cement casing lining and also penetrate a desired lateral depth into the surrounding formation. Explosive well completion devices are also employed which cause explosive movement of projectiles from a perforation tool laterally through the well casing and into the surrounding formation. These perforations permit petroleum products contained within the formation to be channeled into the well casing for production through production tubing to surface located production equipment. Explosive initiated well tools are also employed for the purpose of setting and releasing packers for sealing off between the well casing and production tubing extending through the casing. Packers are often employed to isolate a section of the well casing traversing a production formation to thereby insure that only a limited section of the well casing is pressurized by the production pressure of the formation.
Since the handling of explosives is an inherently dangerous activity, for the protection of personnel and equipment from the adverse effects of undesired explosive detonation it is highly desirable to provide a firing control system that permits firing of the explosive only under strictly controlled circumstances. It is imperative that a downhole explosive device be permitted to fire only when it is properly located at designed depth within the well; otherwise, the well casing could be perforated at the wrong depths or well service personnel could experience significant danger.
At the present time, downhole explosives are set off or fired primarily by mechanical means. An explosive device may be controlled by shear pins which prevent detonation of the explosive until one or more shear pins are sheared through controlled mechanical operation of downhole blast control equipment. Under certain circumstances, a bullet type downhole device may be struck by a suitable firing device for initiation of a detonator that will in turn, achieve first order detonation of the explosive controlled device. As a further alternative, a ball or bar may be dropped within a firing string positioned within the well bore to move a mechanical device to its firing position. The prior art further includes downhole firing control devices that are controlled by pressure, time, and motion. An even further type of downhole explosive control device incorporates a ratcheting system to accomplish release of a spring-loaded firing pin to set off a detonator for the explosive. This type of ratcheting device is activated by using the wireline of a downhole explosive control system to pick up and set down a specific number of times to determine the number of ratchets that occur before the firing pin of the explosive device is released for detonation of the explosive.
The existing techniques for handling explosive devices in the downhole environment are subject to significant disadvantages. If, for some reason, an explosive device is positioned within a well bore and fails to fire or for some reason is not fired, it must be retrieved from the well bore in its unfired condition. Many wells have been seriously damaged when an unfired explosive device is inadvertently fired or fires of its own accord during retrieval from the well bore. This type of undesirable explosive firing is seriously disadvantageous when the explosive apparatus being retrieved is a casing perforation system. In the case of perforating strings or blast joints the well casing can be perforated at an undesired depth, requiring expensive and time consuming repairs. Also, in the event the explosive device should fire inadvertently as it is being removed from the well bore at the surface, it can be a significant hazard to workers and equipment that is located at the surface. It is desirable therefore, to ensure the provision of a novel downhole blast control system that effectively prevents inadvertent firing of an explosive device while being run into the well bore, retrieved from the well bore or handled at the surface during insertion or retrieval.
Another significant problem in the firing of downhole explosive charges is that they are typically controlled by means of wireline equipment for downhole positioning and retrieval. The operating system for actuating the explosive firing device must be manipulated by means of surface controlled equipment with the hope of setting off the detonator with the downhole device accurately located at a predetermined depth within the well bore. Many different types of explosive control devices require the wireline equipment to be picked up and set down a number of times in order to achieve the desired mechanical result, i.e. the firing of a firing pin, shearing of a shear pin, etc. It is often difficult to ensure accurate positioning of a downhole explosive device so that the desired subsurface operation is accurately and safely carried out. It is desirable therefore, to provide a downhole blast control system that effectively ensures accurate positioning of an explosive device at a desired depth within a well bore and selective electronic firing of the explosive device only when predetermined parameters of firing control have been successfully met. It is also desirable to provide a novel electronic blast system that ensures that a downhole explosive device which fails to fire or is not fired for any number of reasons can be efficiently and safely retrieved from the downhole environment without compromising the safety of the well equipment or the well personnel at the surface.
Electronic well control equipment has been developed which utilizes time and motion as determining factors for positioning of explosive equipment at a selected depth within the well bore and for achieving its explosive detonation for accomplishing work. In existing electronic control devices, electronic time and motion responsive signals alone are not considered sufficiently adequate safety features for dangerous devices such as those utilizing downhole explosives for explosive energized well activities. It is desirable, therefore, to provide a downhole blast control system which establishes a plurality of electronic parameters all of which must be satisfactorily met before the explosive device can be electrically initiated. These electronic parameters include hydrostatic pressure signals and well fluid temperature signals which can initiate electronic firing signals only when the detonation control system is located at designed depth within the well bore.
When downhole detonation of explosive devices is accomplished, especially for casing and formation perforation during well completion activities, it is desirable to know the condition of the well bore and production formation before, during and immediately following detonation of the explosive. Heretofore, the condition of the formation has been determined largely by running well data tools into the wellbore to formation depth after the blasting tool has been retrieved. Obviously, after a delay of this duration the production formation will have stabilized so that certain data, such as formation recovery pressure and production rate cannot be immediately determined. It is desirable therefore to provide a novel electronic blast control tool which incorporates a solid state non-volatile memory which is capable of receiving and storing many thousands of well data sets, including time, well fluid pressure and well fluid temperature. After recovery of the blast control tool the memory can be dumped to a computer memory for storage and later processing. These well data sets include well data before, during and immediately following blasting to provide a full range of data that evidences the condition of the formation to be produced.